Optical fibers can be coupled by melting their ends into lenses and placing the lenses in alignment and a predetermined distance apart. A lens can be formed by placing the tips of a pair of electrodes on opposite sides of a fiber end portion and establishing an arc between the electrodes that melts the fiber end portion into a lens. Early experiments by applicant involved the establishment of a DC arc of constant current, at a current amplitude continued for a period of time which resulted in a lens of a diameter greater than 25% of the fiber diameter and preferably about 50% greater than the fiber diameter. While the lens produced in this manner was able to couple light from one optical fiber to another, its mechanical reliability was not as great as hoped for. It was found that the lensed fiber had a weak spot lying a short distance behind the root of the lens, where the fiber was most likely to break. In pull tests, for example, the tensile strength of the fiber immediately behind the lens was only 40% of the tensile strength of the rest of the fiber. Also, multiple microscopic crevices could be observed in the lens surface, which could absorb moisture and develop cracks in the lens.
The use of an electric arc to melt the fiber into a lens was initially patterned on the use of electric arcs in fusing the ends of abutting optic fibers to splice them. In splicing, however, the fiber ends are merely softened with minimum deformation rather than being melted into balls, and in splicing problems are not encountered as to great weakening of the fiber behind the spliced ends or in the development of crevices which can cause damage to the fiber end.
In splicing, an arc of several milliamperes may be applied for a period of perhaps two seconds, to soften the ends of abutting fibers to splice them. An AC arc, which may be of low frequency, may be applied to produce a more uniform arc. When a similar setup was used in an attempt to melt an optic fiber into a lens, but with a higher current, the lensed region was found to be fairly brittle and to exhibit considerable crevices. A system for using an electric arc to melt a fiber end portion into a lens, which produced lensed fibers of high strength and smooth surfaces having a minimum amount of crevices, would be of considerable value.